I have a D50 and when I process my images in Lightroom noise at ISO 1600 isn't that bad really...
My brother has a D80 and although the noise is less, it's not worth £400. If it really bothers you why not buy a second hand D200 and see a real difference? ~£550
Sometimes I keep the ISO down by under-exposing by 1/3 or 1/2 stop, then pushing it back up afterwards. Shooting RAW also helps a lot.
May I ask what sort of situations you are in which require a high ISO?
When used correctly noise reduction software doesn't take much/if any detail from an image.i've always been weary of them, i'd be loosing detail from the image..